Microwave is a type of electromagnetic wave with a frequency generally between 0.3-30 GHz, and an industrial microwave dryer normally operates with microwave of a frequency of 915±25 MHz or 2450±50 MHz. Plasma is produced by breakdown of gas to form electrons and ions through an electric field, wherein the electrons are accelerated by an electromagnetic field such as microwave and collide with the gas to induce more production of ions and electrons, such that electrons and ions are continuously generated until equilibrium of such production and depletion is reached to thereby form stable plasma. Besides a few ions and electrons, plasma also contains highly reactive substances of an excited or transient state and free radicals, which can be used to perform particular reactions that are hardly implemented by basic-state substances.
Microwave plasma technology employs microwave energy to produce plasma, and can be applied to thin-film fabrication, nano-scale materials, wafer etching cleaning and burning of toxic waste gas. Microwave plasma is a plasma reaction system without requiring electrode and heat generator, whereby microwave is introduced as a primary energy source to a reaction chamber and induces gas molecules in the reaction chamber to decompose into atoms and then charged ions. However, in the use of microwave discharge to stimulate gas molecules, not all the gas molecules or atoms readily form ions by means of electron collision; in fact, most of the gas molecules are elevated to be in an unstable high-energy state by electrons, but quickly return to a stable low-energy state.
Microwave plasma is critically utilized for deposition of thin-film materials, fine processing and material surface modification. In terms of benefits such as high ion density and ionization, strong chemical reactivity, satisfactory process reproductivity, and low reaction temperature, microwave plasma is suitably applied to a plasma chemical gas thin-film deposition process and a plasma etching process under low temperature; this feature is important for fabrication of large-scale integrated circuits, microelectronic elements, photoelectric and communication integrated circuits, polymer materials, and thin-film sensors. Furthermore, in the use of a microwave plasma source of electron cyclotron resonance (ECR), gas molecules are accelerated to obtain high energy, and beam current density of an ion beam is increased, as well as processing area of the plasma source can be enlarged; in view thereof, development of microwave plasma advances application of ion source technology. In particular, the microwave plasma technique plays an important role in surface modification of large semiconductor elements, photoelectric materials and polymer materials; for example, a plasma etching process can be used to produce submicron-scale elements and large integrated circuits, plasma can be used to form thin diamond films, and so on.
Microwave can be transmitted in many ways, for example, via a coaxial cable, waveguide or dielectric, wherein the waveguide is required for high-energy microwave. Generally, a microwave device uses an electron tube named magnetron as a microwave source, allowing microwave of a particular oscillating mode to be transmitted via a waveguide, and introduced to a plasma chamber through quartz glass or dielectric windows, so as to collapse gas fed into the chamber for producing plasma by means of an electric field.
Microwave plasma is manipulated to allow microwave produced from the magnetron to pass through a circulator and enter into a plasma promoter, wherein circular metal tubes are connected to upper and lower sides of a square waveguide and each internally provided with a quartz tube; by reducing pressure in the quartz tube to approximately 0.1-1 torr via a vacuum pump, gas can easily ionize to form plasma by means of microwave power. However, the above arrangement would induce significant problems; cooling of the metal tubes by cool water is not effective due to considerably high temperature of plasma. Further, the produced plasma is of limited surface area in compliance with size of the waveguide, making this plasma only applicable to laboratory-scale usage. A solution is to enlarge portions of the square waveguide proximal to the circular metal tubes, in order to accommodate larger metal tubes and quartz tubes; nevertheless, certain limitation is expectably set on size enlargement.
The above small-area microwave plasma is not satisfactory according to industrial requirements; however, due to relatively short wavelength of microwave and complex interaction between microwave and plasma, it is therefore not easy to produce uniform plasma with large surface area. In response, a solution or breakthrough thereto is primarily based on an idea to make plasma in the form of a resonant chamber where microwave energy can be stored for inducing gas to generate plasma under a certain condition.
ECR microwave plasma operates to transfer energy of ionized electrons to atoms or molecules by means of elastic or non-elastic collision to induce a chain reaction for producing plasma. However, in a chamber with low pressure (<10−2 torr), gas molecules are relatively fewer in quantity to rarely collide with electrons, thereby making it hard to form plasma. A solution is development of an ECR microwave plasma device; besides the use of microwave to stimulate gas molecules to ionize and generate plasma, a magnetic field is additionally provided in the chamber, and aligned vertically to an electric field in a manner as to allow electrons to be in a circular motion with a spiral moving path. When a moving angle frequency of electrons is equal to variation of microwave vector, a situation of ECR is reached, which elongates a moving path of electrons and thereby increases chance of collision between electrons and other gas molecules, so as to allow plasma to be formed and maintained under a low pressure condition. Generally, ECR operation pressure is around 10−4 to 10−3 torr; in such low pressure, impurities are less and contamination to raw materials is lower, and an average free diameter of gas molecules is larger, thereby making gas accelerate to obtain higher energy.
ECR is primarily applied to etching in the semiconductor industry, but limited in size corresponding to design of the resonant chamber. Moreover, as permittivity (εr) of plasma is between 1 and 2, when plasma is produced, a condition of a station wave generated in a plasma chamber would also change, such that several adjustments are required. For example, besides using a stub tuner to modulate impedance matching between microwave and plasma, intensity of a magnetic field and microwave output power can also be adjusted; nevertheless, it is still uneasy to form stable plasma as desired.
Particularly, when microwave penetrates through quartz windows and stimulates gas to produce plasma, since certain impedance exists between microwave and the whole mechanism prior to gas stimulation, this impedance would change in terms of many parameters by gas excitation, but can not be manipulated to form stable and uniform plasma. In addition, during coupling between microwave electric fields in the waveguide, microwave reflects to a great extent due to discontinuity of wave impedance for coaxial transmission of microwave directly, and when gas is introduced into a microwave plasma processing chamber, by discontinuity in dielectric coefficient of the device and environment, current accumulates and a plasma sphere can not be formed at a proper or desirable position. Furthermore, for suitably controlling pressure in the plasma processing chamber, an air-extracting system is used to reduce inner pressure of the chamber; however, this air-extracting system usually results in undesirable turbulence to gas provided by a gas supply system, thereby making plasma distribution uneven and degrading quality of fabrication process performances.